GraphQL: Expose current user todos for todoables

What does this MR do?

This MR introduces a new CurrentUserTodos GraphQL interface that can be added to "todoables" to expose todos for the current_user on the object.

The field will return all todos, but accepts an optional state argument of an existing TodoStateEnum value (pending or done).

The specific use-case is #198439 (closed) where we want to discover a user's todo on a Design when querying for it, allowing the user to mark the todo as done using the existing todoMarkDone mutation.

The interface has been added to DesignType, IssueType, MergeRequestType and EpicType.

It hasn't been added to CommitType as the lookup for a todo on a Commit differs from the other "todoables". This could be implemented separately when required.

Example

{
  project(fullPath: <full-path>) {
    issue(iid: <iid>) {
      currentUserTodos(state: pending) {
        nodes {
          id
        }
      }
    }
  }
}

• Issue: #198439 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done